I believe the report predates the unveiling of the DPRK 300mm MLRS.
Counter battery fire is a tricky one to figure out. While I agree that fire finder radars might be overwhelmed with large early salvoes, I also expect those salvoes to be sharply reduced when ROK ballistic missiles start falling. Surely ROK has ballistic missiles that are pre-targeted to hit the DPRK's hardened artillery sites and ready to launch on short notice. More mobile NK artillery would probably escape such a fate, but they are probably shorter ranged as well to facilitate transport. I also tend to think that PGMs mitigate the issue of aircraft dodging artillery shells since those munitions can glide to their targets and maintain accuracy from high altitude. It still remains a concern for mobile targets that must be found and lased however.
While a surprise artillery barrage may catch the ROK with their pants down, any sort of moderate military buildup would certainly be detected considering how closely watched the border area is. Even if troop formations stay in Pyongyang, I wouldn't be surprised if the US and SK pick up on stockpiling of fuel and munitions in anticipation of an artillery attack. Whether we would prep aircraft and brief pilots based on a build up without infantry formations I don't know.
I don't know of a good solution to the issue of Chems hitting Seoul. You have a good point about underground shelters being death traps for Chems. I doubt that many of the shelters in Seoul are rated for CBRNE since most of them are probably subway stations and basements. The main thing (hopefully) restraining the DPRK from deploying Chems is the threat of US nuclear retaliation.